TURBO Q & A FORUM

Post by **510rob** » 23 Oct 2003 17:58

WASTEGATES...

A few basics to start with

...the pressure ratio of a centrifugal pump increases at the SQUARE of RPM, this is about as non-linear as you can get!

...a turbocharger is inherently a positive feedback device; all positive feedback systems are marginally stable at best, and completely unstable at worst.

...a positive feedback system with a non-linear response is, ummm, well, pretty crappy to deal with on its own, and mostly useless without some form of negative feedback stabilization applied to it.

...guess what the wastegate provides! Negative feedback stabilization of marginally stable and unstable turbo systems!

You have a few ways to go on the choice of exhaust housing...

#1 - Go waaay too big... You will never have enough energy to really get the turbine moving, so it won't boost, so it won't increase exhaust flow, so it will never do much, so its kind of stable, but the possibility is still there, given enough RPM...

#2 - Go kinda big... At a certain point, there will be just enough exhaust flow to wake up the exhaust wheel, and that will spin the compressor wheel just enough to blow a little bit more air into the motor, and that will feedback to the turbine (exhaust) and all of a sudden, boost will jump like a friggin' MONSTER, and the motor will then try to rip the car in half, or the driveshaft, or whatever it can, or just fry the tires...

#3 - Go kinda small... There will be a decent amount of energy to spin the turbine, so it will start spinning at midrange rpm pretty well, and the car will be pretty kick-ass, but the top end flow will be choked off a bit because of the slightly small turbine housing's slightly lower flow capacity... sacrifice a bit of top end power for some mid-range driveability

#4 - Go waaay too small... There will always be lots of energy to spin the turbine, at almost any RPM - even if you aren't even on full throttle (this is annoying, and the DSM stock turbos tend to do this dumb trick) - A really small exhaust housing makes the car feel jumpy and powerful to dumb people who don't understand what a turbo is supposed to do - KICK ASS!!! Problem is, they never realize that they've cut themselves off at the knees with the small housing because it will never flow the horsepower numbers they dream about... but it reacts fast, so when the stab the throttle at 2000rpm when they are "hot-footin' it", it "feels" quick, but when they try to get the motor to rev from 6000rpm to 7000rpm, it takes about 10 minutes because there is that damn tourniquet around the exhaust wheel...

#5 - Go "just right"... Well, there is no "just right" size, everyone's tastes are different;
some people like punchy midrange (kinda small)...
some people like the nuclear top end hit (kinda big)...
some people hate lag (waaay too small)...
some people try to use a Mack truck turbo (waaay too big)...

So, yeah, anyway, let's get back to the wastegate B.S....

If you use a Mack truck turbo, it is still marginally unstable by nature, but you'll never force any of the mathematical poles close to zero or into the two positive quadrants. What the hell?!?!? In non-geek-math speak, that means, it is soo friggin' big that its marginally unstable nature will never be a concern mostly because it isn't gonna do much of anything but weigh about 80 pounds...

Once the turbo gains the ability to start getting some boost within your rpm range, then it becomes a risky as an unstable system without feedback control - A WASTEGATE, or some other form of restriction that will provide stability by limiting the poles to the two negative quadrants - if the exhaust is 1.5" in diameter, it isn't gonna be very easy to run 400HP through it... so the turbo just isn't gonna flow it, right? Same thing goes for restricted intake systems... you can't suck 400HP worth of air through a single 1" hole, right?

So here's the "something for nothing" nature of a wastegate... you can run a smallish exhaust housing that will get the turbine spinning at a reasonably useful RPM range, and the wastegate will regulate the intake manifold pressure by varying the driving pressure applied to the turbine wheel... if you didn't have the wastegate, and you floored the accelerator pedal, the turbine would do giga-rpm, and the poor old attached-at-the waist compressor wheel would be forced to attempt to blow giga-air into the motor in a positive feedback loop of destruction, and either the motor or the turbo would kill itself in the process (most likely both of them would die...)

A big wastegate, or a small one??? This depends on a couple of things... for a given turbo and motor, the more boost you run, the SMALLER the wastegate you need! For a given motor and a given intake manifold pressure, the smaller the exhaust housing you run, the BIGGER the wastegate you'll need TO MAINTAIN TOP END CROSSOVER FLOW... at high RPM, the exhaust backpressure between the valve and the turbine can easily climb higher than the intake manifold pressure, and that's where the motor starts to make no power, detonate from heat-soak, and kill itself and burn the valves... and this is where the bigger turbine housing comes into play...
if you try to run a smallish turbine housing, a smallish wastegate, low boost, and very high rpm, you might begin to see "boost creep" = the boost starts to rise at high rpm because the wastegate can't bleed enough energy around the turbine wheel...

Are all wastegates "adjustable"? Basically, yes - you can change the spring to set the range, or crank up the preload screw, or shim the spring to increase the boost pressure, or you can get a small air regulator and "fool the wastegate" (cockpit adjustable boost control), or you can spend a bunch of money on a flashy boost controller... these things do have their place, but not in my car...

Robert F.

P.S. - 5 minute cool-down discussion to follow...

Post by **bertvorgon** » 24 Oct 2003 19:21

Wow, Rob, that was good. I'm not sure where I fit in on that one. It just goes REEEL GOOOOD> I ALWAYS let my motor idle for a minute or two after coming to a stop. If I have not been beating on the car, I let it idle for 20 secs. or so. At the Knox hillclimb I would let it idle for a good two minutes because you come across the line at full throttle, then are stopped. Even at that I would re-start the motor to move cool water from the rad into the block, and flush fresh oil through the turbo. That worked well for all those years. I looked at some of stupidly priced boost controllers. Nuf said. I have that pressure regulator beside the seat and it has worked just fine. Sorry you cannot make it Spencer, you have a rain check on that.

Post by **5teN** » 25 Oct 2003 01:45

Awesome info once again guys! Thank you!
Hrmm what else can I ask...

Post by **510rob** » 25 Oct 2003 11:34

LETTING THE TURBO COOL DOWN FOR 5 MINUTES, TURBO TIMERS, AND OTHER THINGS LIKE SYNTHETIC OIL...

To start, the brief answer is, if you have been really lead-footing around for a while, it is very advisable to putt-putt around for a few minutes before you shut off the motor; this allows the nuclear-hot turbocharger cool down before you shut down the motor, and consequently shut off oil flow to the turbo. If you don't let the turbo cool down, it could be well over 900 degrees, in which case you've got yourself a mini distiller for cracking crude oil into its various light aromatic components... WHAT???

OK, now we have to look at what oil is, where it comes from, and all that crap...

You start with tar-pit-like crude oil, unless you are lucky enough to strike a clean deposit like the Beverly Hill----billies, "that is"... you put it into a furnacey barbequey kinda vertical boiler, heat it up slowly, and as the temperature rises, various components start to evaporate at different temperatures...

A good example would be a solvent tank... how in the hell do you think they clean the used solvent/toxic waste?!? They put it into a distiller, and as the temperature is raised, the various solvent components, like acetone, or xylene, or toluene, or whatever the hell is in there (I don't really know, so don't pin me on this one) will evaporate at different temperatures, so if you hold it at the xylene boiling temp, collect the gaseous emissions, condense it, you will end up with pure xylene!!! cool!!! then they run it through a fine filter and sell it back to you as new clean solvent!!!

ok, so back to oil... when you boil crude oil, you get methanol at one temp, parrafin at one temp, gasoline at one temp, and octane, and cetane, and heptane, and ...blah blah blah = all of the other hydrocarbon molecule chains of different lengths and consequent weights/boiling points/energy levels... you know when you leave your jerry can in the sunshine and it gets REALLY bulged? That is evaporation of the lighter aromatic hydrocarbons... if you read the VP specs, they rely on as few
of the aromatics as possible, because they evaporate too easily - they say to keep their particular witch-brew out of the sunshine so the tetra-ethyl lead component won't oxidize and break down - different reasons, same problem - gas goes shitty if left in the sunshine, and the best small gas cans are still those crazy old WW2 German steel cans (they really knew what they were doing!)

ok, so back to the main subject again... if the oil breaks down into different components at different temperatures, and the light stuff goes first, then once you've boiled off the light stuff, you are left with heavier components, like lubricating oil... so if you try to crack the heavier stuff, guess what you get? sludgey crap, and if you cook it hot enough, solid "coke", which is basically like burned junk useless crap byproduct of the refining process... coked oil is like slag from the steel refining process; nobody really wants it, or wants to deal with it...

When oil cokes, it becomes a crusty burned mess. When the oil cokes in the turbo (a turbo is more than hot enough to coke oil in certain circumstances), the oil crusts and gums up in the turbo's very small oil passages and screws up the proper oil delivery for those poor bearings that are trying to cradle the shaft that is spinning up around 100krpm (if anyone uses the old 220krpm bullshit, that turbo was too small to begin with!). You on'y have to starve the oil flow once to fry a turbo bearing if you are racing (for lack of a better term).

So, yeah, turbo temperatures can be all over the place, based on engine loading and power output... if you drive around at full throttle, the turbo will be hot enough to coke oil, if you drive around at low speed and low power output, the turbo will still be very very hot, but probalby won't coke the oil as badly... the oil cools the shaft and bearings, and can pick up about 80 degress F as it passes through the turbo... mineral-based oil starts to break down at about 300 F, so if you have oil temp of 240 F , and put it through a turbo, you are starting to "refine" out some of the oil's components... uh oh... CHANGE THE OIL OFTEN!!!

...if you switch to synthetic oil, most of the synthetics are processed vegteable based molecules that have been turned into frankenstein super oil in big lab refinerys, and what you end up with, to make a REALLLY long story a bit less long, is an oil that starts to break down at ...about 400 F... Ah HAH!!! so there is some reasoning to use it in a turbo motor after all!!!

ok, so back to the main subject again...
3 Factors determine oil coking
* oil residence time
* localized heat flux
* oil mass flux rates

ummm, YEAH, ANYWAY, if you have a steady stream of oil, its not really going to coke, but it will break down a bit with the heat... BUT! as soon as you interrupt the oil flow (shut off the engine) there is no stream, so the oil stops moving - "oil residence time" = the oil is sitting there in the turbo bearing housing, and the shaft temperature is
* REALLY HOT hot if you've been ripping it up
* plain old hot if you've been a law-abider

So, you should give whatever oil you have in the turbo a chance, and that is done by letting the turbo cool down after a session of "hot laps", and that is done by maintaining the flow of oil, but at low engine power output levels... like putt-putting around for a few minutes, or by ...idling for about 5 minutes...

And there you have the basis for the "turbo timer" which will let your motor idle for a number of minutes in your absence, which I think is absolutely crazy - what if something goes wrong, and you are not there? Do you think the turbo timer is listening for a rattling rocker arm, or whatever else?!? I have read that Australia recently banned the use of turbo timers because it is a violation of their motor vehicle act in that the car is running unattended; if you start the car, and go back inside the house to grab something, you are intending to return I suppose, but if you hit 5 mintues on the turbo timer and walk away, who know when you'll be back to find the turbo timer that malfunctioned, and there's your burned-out mess sitting there that used to be a source of pride, all black and charred, surrounded by a bunch of pissed off firemen, all glaring at you, holding those shiny red rescue axes with the sharp little point on the back... uh oh!

...and they all lived happily ever after

Robert F.

Post by **bertvorgon** » 27 Oct 2003 21:13

Hello Rob,
On Sunday, I took the car for a good boot up the Hope/Princeton. After reading your article on wastegates etc. I thought I would watch to see if the boost crept at all, once it was up there. It stayed fully stable at 16 LBS.. The engine pulled very hard to 7,500 RPM. and felt like it would keep going, albiet not breathing a well as the last cam did at that RPM. Do you think that the lag will be that great when I go to the 1.1 housing? Do temperatures usually drop with the bigger housing? Would I see an exhaust temp drop, or is that still a function of combustion? When I look at the Boost / Airbox pressure drop, it appears to be only 1-2 LBS across the carb. Is there a magic number that is best/worst?

Post by **510rob** » 28 Oct 2003 18:10

greater lag with bigger housing?

The rpm threshold at which the motor starts to make boost will move up the rpm band a bit (resulting in a bigger hole in the lower-mid RPM). Also, you should notice a more abrupt transition from low boost to to full boost once the wheels really start turning (less backpressure between exhaust valves and turbine wheels means greater mass flow). In addition to those, you should have stronger upper-mid, and top end power. If you keep the RPM's up (where the motor is already making boost), there shouldn't be any increase in pressure delay because you haven't changed the mass of the rotating assembly. These things only apply at RPM where you are capable of making boost with the larger housing... below that RPM, things are a totally different story, and the motor will feel like a slightly restrictred version of its aspirated evil-twin self, then it will come on harder and more abrupt, and more powerful...

do temps drop with larger housing?

You probably won't see a super significant temp. drop, but with the increased flow capacity of the larger turbine housing, there will be a reduction in backpressure which will improve the ability to exhaust hot gasses to get through the motor, thereby dropping all associated temperatures a bit, but not like 200 degrees or anything, I would suspect 50 degrees perhaps?

exhaust temp as a function of combustion, or housing?

Exhaust temp is also a function of the type and characteristics of the fuel you are burning, so it is really hard to say what effect the larger turbine housing will have on exh temp (with any lab accuracy) once the motor is properly set up for the new housing. I would stick with my last answer and conclude that the potential increase in exhaust flow makes the motor inherently more volumetrically efficient, and that requires (or better "allows") the combustion of more fuel, so now you can burn more fuel at a better efficiency, so you have created more heat, but at a better efficiency, so is it any hotter?

pressure drop across the carb?

I was inferring total delta pressure across cylinders... there's not much point in running 30psi intake when it gets you 35psi exhaust manifold pressure - that's when things start to get waaaaay too hot all of a sudden, and you make no power...

Robert F.

(I.m kinda dozy, so if there's mistakes, blame it on my lack of good sleepies lately)

Post by **bertvorgon** » 28 Oct 2003 19:58

Thanks Rob, that is kind of what I thought. I cannot imagine that it may "hit" actually harder than it does now. When the secondary opens it just blows you back into the seat. Maybe because it is happening at a relatively lower RPM. I have a pretty good sense of what it will be like. A temp drop of 50 F would be just fine, it all helps.
I can still see Andy's Colt when it would get up on the boost...that WAS impressive ( 400-450 HP) I hope mine is NOT that good...my tranny will not like that much torque. I think it is getting marginal now, which is a worry as parts are not available for that comp. box. Andy thinks we should be thinking of putting a Supra transmission in, that would hold the power. We will see what next summer brings.

Post by **510rob** » 28 Oct 2003 21:25

Do you still have the 0.87 turbine cover on there right now with the 229 compressor?

I remember talking to Noel about the Colt's "prototype Mitsubishi" (...never you all mind what it really is - that still might be top secret!!!) motor once; he said Andy was running something like 28psi intake pressure and about 9000 or so RPM, and getting a c _ _ t hair over 500HP... I only saw it at Westwood twice; the first time I saw it race, I didn't know what the hell it was, other than... CCCCCOOOOOOOOLLLL!!!, the second time I saw it "race" was the infamous outing where the prick running the flagtower made everyone do a 3-lap warm up that caused the Colt to BADLY foul its plugs, so Andy pulled into the pits and left in disgust...

I remember him coming around and out of the hairpin, before literally shooting up the hill, and the car sounding kinda like this... kkkaaakkkaaakkaaakkaaaaAAABAAAAWWWWAAAAAAAAAAWWWWOOOOOOSSSSHSHHHHHHHHHHSSSSSSSSSSSSSSSSSSSssssssssssssss.....................launched into orbit!

Post by **bertvorgon** » 29 Oct 2003 07:15

Yes, I have..We believe a 229 compressor??!!! That is what the tag says when I got it back from the re-build. And the .87 housing. As we discussed on the phone, I did not know that was available. I have an option of having a complete turbo set up with my bigger housing. This way I would just have to change the turbo out to get the different turbine sizing. It is just as much work to pull the turbo just to change an exhaust housing.
You summed up Andy's car just fine. It had that injection system that Chris Dumont built on it. It had ISSUES at low speed thats for sure. Fouled plugs was one of them. He actually only ran about 18 PSI.. He also ran quite high static compression also. His methonol tank was the size of most peoples fuel tank! Do you have a compressor map for that 229?

Post by **510rob** » 31 Oct 2003 17:43

I only have the Schwitzer maps from the old Crane turbocharging book (the Don Hubbard book or something like that?!?)

I think the maps are for the 168, 198, 271, and 305 compressor trims... no map for the 229 - I think it was a much later introduction to the 3LD lineup than that book's publishing date, and by then I guess the deal with Harvey Crane was off!!!

Now that you've got the intercooler, 3" exhaust, functional blow-off valve, and a fresh motor with stronger bits in it, I think you should try the 1.10 turbine cover for kicks... see how much top end it gives you... you'd probably drop about 500 rpm off the low-end response range, but get a bit on the top end in trade... as the saying goes, "pay me now, or pay me later..."

DOES ANYONE HAVE A G-TECH OR WHATEVER THEY ARE CALLED SO KEITH CAN DO SOME HYPER TESTING??? (Ooops, I meant Dr. Bert Vorgon from Siberian District Lab, Gravitron Brakeski Building, Glasnost Research Facility #510, High Acceleration Airflow Testing Project - sorry for compromising your top-secret cover, comrade)

RF

Post by **5teN** » 01 Nov 2003 01:15

I have access to a Gtech (one of the first ones)...And I'd love to do some testing

Post by **bertvorgon** » 01 Nov 2003 11:55

The GTech would be interesting. Likely find out I only have 150 HP and does the 1/4 in 18 secs. Been a long time since Bert and his brother Rimsky have done any testing, and ended up in Lake Bullshitski. I think the last time was with the Lada. I took James McMillan out on Thurs. night for a ride. I think he enjoyed himself. We hit 7,550 RPM in 5th on HWY. 99. No traffic of course. I could loose 500 RPM off the bottom no problem, this thing hits sooo hard now I am worying about my transmission. Big housing will be OK I think. Car is being put away for the winter now. I will post proper engine storage procedures when I'm done.

Post by **jovial_cynic** » 08 Nov 2003 10:42

i plan to actually read a book on turbos and whatnot, but i figured i might as well get this answered now so i can start making plans one way or the other.

i finally got my naps-z 2.2e motor into my 510, but it still needs to be wired up, and i've got to get the extra fuel line connected...

i happen to have a turbo from an 84 nissan 300zx, but since i'm unfamiliar w/ the output of the turbo, and unfamiliar w/ the tolerances of the naps-Z 2.2e under boost, i have NO idea if this turbo is appropriate for my engine. if it's not a good match, i'll just sell the turbo and look for something else. but if it is, i'll consider working towards the turbo once i get the car up and running.

so... any ideas?

Post by **510rob** » 09 Nov 2003 13:11

I can't find the part number, so you'll just have to tell me what it is...look on the turbo for a number like 464852-5 or something like that (the Garrett number, not the Nissan number!!!)

first off, you can most likely measure it up to figure out what it is...and if it will work based on flow maps...

What is the A/R number for the compressor cover, and what is the A/R number for the turbine cover? - There should be a number cast into the turbine cover just inside the mounting flange ("should be") or cast on the outside somewhere , like 0.48 or 0.63 or 0.60 or something...

if you pop the covers off the turbine and compressor, measure up the two wheels for their inducer and exducer diameters (the large diameter, and the small diameter)

we can go from there...

in short, it shoud be fine, and if you are thinking it is gonna be too big... in my opinion it is way too small for the 3.0 to start with, so there you go...
it should be a good start for a really fun street setup that will burn most cars that aren't full-sleeper prepped...

Post by **jovial_cynic** » 09 Nov 2003 16:40

well, off hand, i know that it's a garrett T-3 turbo, but there are a few variations of the T-3: the 40, 45, 50, 60 trim, and super 60 trim. The air-flow maps are found here:
http://www.turbofast.com.au/FlowT3.html

i'll have to wait until i get home to actually see the specific numbers on mine, but that's what i've got so far.